The issue presented is whether certain officers in the regular police force of the city of Lawrence (city) met the three-year employment requirement necessary to take a promotional examination for the position of sergeant pursuant to G. L. c. 31, § 59.

*310In 2000, the human resources division of the Commonwealth (HRD) announced a departmental promotional examination for the position of police sergeant in the city’s police department. In a letter dated October 4, 2000, the city’s mayor requested that HRD declare that ten officers listed in her letter were eligible to take the exam. The mayor wrote further that they had “performed all the duties, faced all the dangers, and made all the sacrifices incumbent upon a Lawrence Police Officer.” She also included their payroll records to show that four of the officers had worked full-time with uninterrupted service since December 21, 1995, and that six of the officers had worked full-time with uninterrupted service since December 23, 1996. They were not, however, given “ ‘permanent’ civil service status” in the regular police force until December 15, 1997.2 The HRD determined that the ten regular force police officers had the necessary three years of experience preceding the examination date (October 21, 2000) and therefore included the officers on the list. One of the officers, James J. Raso, received the promotion.3 The appellants, nine other officers of the Lawrence police department, challenged the inclusion of the ten officers on the list.

The Civil Service Commission (commission) ruled that the officers listed in the October 4, 2000, letter did not meet the statutory requirements to take the exam and therefore that Officer Raso’s promotion must be vacated. The commission interpreted G. L. c. 31, § 59 (see infra), to require that an applicant have at least three years of full-time service in the regular force in order to be included on a list to take the *311examination for promotion to sergeant. The commission concluded that prior service in the reserve force could not be considered unless the number of police officers with three years of service in the regular force was certified by the appointing authority to be insufficient to allow adequate competition for the position. The city appealed to the Superior Court pursuant to G. L. c. 31, § 44, and G. L. c. 30A, and after a hearing, the city’s motion for summary judgment was allowed. A memorandum and order allowing the city’s motion and modifying the decision of the commission issued on June 15, 2005, and was docketed the same day.

We conclude that the commission’s interpretation of G. L. c. 31, § 59, is erroneous and therefore affirm the order of the Superior Court.4 Our review of the commission’s conclusions of law is de nova. See Andrews v. Civil Serv. Commn., 446 Mass. 611, 615 (2006), citing Raytheon Co. v. Director of the Div. of Employment Sec., 364 Mass. 593, 595 (1974). As the commission’s “conclusion [in the instant matter] was an error of law,” it is not accorded the substantial deference typically afforded the commission. Mayor of Lawrence v. Kennedy, 57 Mass. App. Ct. 904, 906 (2003). See Boston Police Superior Officers Fedn. v. Labor Relations Commn., 410 Mass. 890, 892 (1991).

General Laws c. 31, § 59, as amended by St. 1989, c. 174, provides, in pertinent part:

“Original and promotional appointments in police ... forces of cities and of such towns where such forces are within the official service . . . shall be made only after competitive examination except as otherwise provided by section sixty and by sections thirty-six and thirty-six A of chapter forty-eight.

“An examination for a promotional appointment to any title in a police . . . force shall be open only to permanent employees in the next lower title in such force . . . provided, further, that no such examination for the first title above the lowest title in the police . . . force of a city *312or town with a population in excess of fifty thousand shall be open to any person who has not been employed in such force in such lowest title for at least three years after certification.[5]

“Persons referred to in this section as being permanent employees in the lowest or lower title shall include only full-time members of the regular force and shall not include members of the reserve or intermittent police . . . force . . . unless the appointing authority certifies to the administrator that the number of permanent full-time members of the regular force is insufficient to allow adequate competition in an examination and the administrator determines that the circumstances warrant opening the examination to permanent members of the reserve [or] intermittent . . . force, as the case may be. Upon the request of the appointing authority, the administrator may include service actually performed while a permanent member of a reserve [or] intermittent. . . force in computing length of service required for admission to an examination for promotional appointment to the first title above the lowest title. The appointing authority shall submit with such request payroll records proving that such service was actually performed. For purposes of this section, two hundred and fifty days, or the equivalent thereof, of such service shall be equivalent to one year of service on a full-time basis in such regular force.”

It is undisputed that (1) the promotion at issue, for the position of sergeant, was for the “first title above the lowest title”; (2) the city had a population in excess of 50,000; (3) the number of officers in the regular force with three years of experience in that force was sufficient to allow adequate competition for the sergeant’s examination; (4) each of the challenged officers was a permanent member of the regular force but had not been serving in the regular force for three years at the time of the *313promotional examination; and (5) each of the challenged officers had been a police officer in the city’s police force for more than three years prior to the examination when the officer’s years of service in the city’s regular and reserve police forces were combined. The sole issue presented is whether the HRD can, upon the request of the appointing authority (here, the mayor), combine an officer’s full-time service in the regular and reserve forces to satisfy the three-year requirement or whether such “tacking on” is authorized only when there is a certification by the appointing authority of an inadequate number of otherwise eligible officers.6 This is an issue of first impression.

We interpret G. L. c. 31, § 59, as providing two means of including on the eligibility list for examinations for promotion to the position of sergeant (the “first title above the lowest title”) officers other than those with three years of experience in the regular force. In the first sentence of the third paragraph of § 59 (first proviso), the Legislature has provided a means by which police officers who are not on the regular force, but are instead on either the reserve or intermittent police forces, may be included on the list. This exception may only be invoked when “the number of permanent full-time members of the regular force is insufficient to allow adequate competition in an examination and the . . . circumstances warrant opening the examination to permanent members of the reserve [or] intermittent . . . force.” The first proviso does not, either expressly or by implication, apply to permanent members of the regular force. Rather, it concerns only the opening of the list to applicants in the reserve or intermittent force. The absence of any reference to regular force employees with less than three years of experience is conspicuous.

The second sentence of the third paragraph (second proviso), however, allows the administrator to include service actually performed while a permanent member of a reserve or intermittent police force in computing length of service required for *314admission to an examination for the position of sergeant (the first title above the lowest title). In contrast to the first proviso, the second proviso is directed at permanent full-time officers in the lowest title in the regular force who lack three years of experience in the regular force but who were previously permanent full-time members of the reserve or intermittent force. There is no express requirement in the second proviso that there be an insufficient number of permanent full-time officers in the regular force with three years of experience in that force to allow this tacking on of experience. Nor do we necessarily link the second proviso to the first proviso, and its requirement of a certification of inadequate competition, where the first proviso makes no mention of regular force employees. Finally, as the challenged officers had three years of experience performing all the duties of regular force officers, there is no question that the substance of the three-year experience requirement was satisfied.

We conclude that the commission made an error of law by confusing the requirements applicable to including reserve force officers on the list with the requirements applicable to including regular force officers with less than three years of experience in the regular force. Although a list may not include reserve officers unless the list is otherwise insufficient to allow adequate competition, the list may include, upon the request of the appointing authority and the provision of appropriate documentation, regular force officers with less than three years of experience in the regular force by allowing them to tack on full-time service in the reserve or intermittent force.

The order dated June 15, 2005, allowing the city’s motion for summary judgment and modifying the decision of the commission is affirmed.

So ordered.